: 85-90 EFFECT OF ROOTSTOCK CULTIVAR ON RIPE FRUIT QUALITY J. Dixon, T.A. Elmsly, F.P. Fields, A.C. Greenwood, D.B. Smith, H.A. Pak and J.G.M. Cutting Avocado Industry Council Ltd., P.O. Box 13267, Tauranga 3110 Corresponding author: jonathandixon@nzavocado.co.nz ABSTRACT The amount of ripe rots on avocado fruit can be influenced by many pre-harvest factors. In Australia, it has been claimed an overriding factor is the rootstock where the amount of post harvest rots is greater with Mexican race rootstocks as they are the most susceptible to anthracnose infection on their leaves. For this reason, 'Hass' avocado fruit from trees with 'Zutano' rootstocks, the most common rootstock in New Zealand, are claimed to have poor fruit quality as the 'Zutano' rootstock has predominantly Mexican race genes. To determine if the rootstock has an overriding influence on New Zealand 'Hass' avocado fruit quality, a preliminary study compared 'Hass' fruit from trees of different rootstocks across several orchards. There were significant differences in the incidence and severity of stem end rot and brown patches within orchards in a particular year that may have been due to the rootstock. The best rootstock each year was not the same across orchards. Where 'Hass' trees were grown on rootstocks of similar parentage there were significant differences in the incidence and severity of ripe rots after storage. Therefore, apparent rootstock differences may reflect more on orchard factors that lead to greater ripe rots. It was concluded that the rootstock of the tree is unlikely to have an overriding effect on fruit quality but may partially explain differences in ripe rots of fruit. This is in contrast to the important role claimed for rootstocks in Australia. Keywords: Mexican, Guatemalan, Zutano, Reed, ripe rots INTRODUCTION New Zealand avocado fruit, when ripening, can develop postharvest rots from a complex of at least five fungi (Everett et al., 2003) the incidence and severity of which are thought to be set on the orchard. There are many pre-harvest factors that influence the amount of ripe rots on avocado fruit. In New Zealand, research has shown that the fungicide spray programme (Everett and Pak, 2001), fruit mineral content (Everett and Pak, 2001; Thorp et al., 1997) and rain before harvest (Pak et al., 2003) can affect the amount of ripe rots fruit that will develop. In Australia, it has been claimed that the rootstock of the avocado tree has a major influence on the amount of post harvest rots that avocado fruit develop when ripe (Whiley et al., 2007). The mechanism of rootstock influence has been postulated as a physiological incompatibility between the different avocado races, Mexican, Guatemalan and West Indian (Whiley et al., 2007). The differences in physiology lead to different fruit mineral profiles, change the starch accumulation profile (Lahav and Whiley, 2001; Whiley et al., 1997) and leaf anti-fungal diene levels (Willingham et al., 2001). These changes lead to greater or lesser crops (Smith, 1993) and low fruit calcium levels that are also associated with increased ripe rots (Hofman et al., 2002; Willingham et al., 2001). Mexican race avocado cultivars have been shown to be the most susceptible to anthracnose infection on their leaves followed by Guatemalan race cultivars with the West Indian race cultivars the most resistant (Whiley et al., 2007). Based on the above information it has been claimed that 'Hass' avocado fruit from trees with 'Zutano' rootstocks, the most common rootstock in New Zealand, will have poor fruit quality as the 'Zutano' rootstock has predominantly Mexican race genes. It was recently confirmed in New Zealand that 'Hass' trees with leaves with high levels of anthracnose infection also have fruit with high levels of anthracnose infection (Everett et al., 2003). Therefore it is possible that the rootstock of a tree is affecting the amount of rots the fruit develop when ripe. To determine if the rootstock has an overriding influence on New Zealand 'Hass' avocado fruit, 85
quality a preliminary study was conducted comparing 'Hass' fruit from trees composed of different rootstocks within several orchards. MATERIALS AND METHODS In 2004 and in 2005, 'Hass' avocado fruit from trees with different rootstock cultivars were harvested from commercial orchards located in the Bay of Plenty and Northland regions of New Zealand. Orchards A and E were located in the Bay of Plenty and orchards B, C and D were located in the Northland region. In 2004, 20 ungraded 'Hass' fruit, 4 fruit from each of 5 trees, from each rootstock on each of the orchards were placed into storage. The harvest dates were from 3/11/2004 to 15/11/2004. In 2005, 100 ungraded 'Hass' fruit, 20 fruit from each of 5 trees, from each rootstock on each of orchards were placed into storage. The harvest dates were from 14/12/2005 to 10/1/2005. There was only one harvest per orchard. Within 24 hours of harvest, fruit were packed into trays and then placed into a commercial coolstore maintained at 4 C ± 0.5 C, 85% RH for 28 days. After removal from storage the fruit was ripened at 20 C ± 1.0 C, 65% RH and the fruit quality assessed at eating ripeness. Ripeness was determined by firmometer when the fruit reached a softness reading of 85 using a 300g weight or by hand feel after calibration to a firmometer. Fruit were assessed for disorders according to the Avocado Industry Council Assessment Manual (Dixon, 2003). Where there were fruit from 2 rootstocks for evaluation the results within each year for each orchard were analysed using a two sample t-test assuming unequal variances. For more than 2 rootstocks within an orchard the results within each year were analysed using a One Way Analysis of Variance using MINITAB version 13.31. RESULTS There were significant differences in the incidence and severity of stem end rot and brown patches within orchards in a particular year that could be associated with the rootstock of the tree (Table 1). The best rootstock each year was not consistently the same across orchards. Fruit from Zutano rootstocks had the least ripe rots in orchards A, C, and D but had the greatest ripe rots in orchard B and E (Table 1). For the two orchards C and D the fruit harvested from trees with Reed rootstocks generally had the greatest amounts of ripe rots. The incidence and severity of ripe rots from trees in orchard C that were 6 years old was not significantly different in the incidence of sound fruit between other 6 year old rootstocks (Table 1). By contrast the fruit harvested from trees in orchard C that were 16 years old were different with the fruit from Reed rootstock trees having a greater incidence of sound fruit in 2004 than fruit from Zutano rootstock trees (Table 1). In 2005, the fruit from Hass rootstock trees had a greater incidence of sound fruit than the fruit from Reed rootstock trees. DISCUSSION The orchards used in this study had different combinations of 'Hass' on rootstocks that allowed comparison of the influence of two avocado races, Mexican and Guatemalan, on the development of ripe rots when ripe. The results have been interpreted within each orchard and only general conclusions have been drawn across orchards with respect to rootstocks and fruit quality. Where there were 'Hass' trees grown on rootstocks of similar parentage, M x G in orchards A and B, there were significant differences in the incidence and severity of ripe rots after storage but the differences were not consistent from year to year. Therefore, on orchard factors that lead to greater ripe rots can be important in setting the level of rots fruit develop. These factors can include: tree age, inoculum levels, fungicide spray programme, crop load and fertiliser programme anyone of which may override a rootstock effect on fruit quality. The fruit from orchards C and D were from trees of different rootstock parentage where Guatemalan and Mexican rootstocks could be compared within each orchard. There was no consistent pattern across years in each orchard of 'Hass' fruit from 86
Table 1. Incidence and severity of stem end rot and brown patches and the incidence of sound fruit using a disorder threshold of 5% of Hass avocado fruit grown on different rootstocks within each year of assessment. rot patches fruit 2 Inc % 3 Sev % Inc% Sev% Inc% 1 Orchard Year Rootstock Race Stem end Brown Sound A 2004 Fuerte M x G 20.0 1.9 45.0 1.7 85.0 Zutano M x G 5.0 0.2 30.0 0.7 95.0 2005 Fuerte M x G 3.0 0.1 32.0 1.2 91.0 Zutano M x G 7.0 0.2 4 14.0** 0.3* 96.0 B 2004 Vista M x G 15.0 0.2 25.0 0.8 95.0 Zutano M x G 20.0 0.7 40.0 4.4 80.0 2005 Vista M x G 4.0 0.1 20.0 0.4 98.0 Zutano M x G 12.0* 0.5 28.0 1.2** 90.0* C 2004 5 Hass 6y G x M 6 45.0b 5.6ab 80.0b 1.9b 65.0ab Reed 16y G 15.0b 1.1b 40.0a 0.2b 95.0a Reed 6y G 55.0ab 3.0ab 70.0ab 2.4b 55.0b Zutano 16y M x G 90.0a 9.0a 90.0b 11.4a 15.0c Zutano 6y M x G 25.0b 1.7b 60.0ab 0.5b 80.0ab 2005 Hass 16y G x M 19.2c 2.4b 60.6 0.7c 80.8a Reed 16y G 44.0b 9.5a 71.0 2.3bc 59.0b Reed 6y G 71.0a 16.2a 76.0 5.2a 40.0c Zutano 6y M x G 55.0ab 12.0a 70.0 2.8b 51.0bc D 2004 Reed G 75.0 3.9 90.0 8.4 60 Zutano M x G 60.0 2.5 85.0 4.2 60 2005 Reed G 39.0 1.9 54.0 3.4 67 Zutano M x G 25.0* 1.7 26.0*** 2.0 82* E 2005 Hass G x M 1.8b 24.0b 1.1b 21.0b 83.0ab Hayes M x G 0.3b 13.0b 1.4b 32.0b 92.0a Hopkins M x G 1.2b 27.0b 4.6b 61.0a 75.0b Zutano M x G 5.1a 56.0a 11.7a 50.0a 51.0c 1 G x M = predominantly Guatemalan Mexican cross, M x G = predominantly Mexican Guatemalan cross, G = 2 3 4 5 Guatemalan; Incidence; Severity; Significance levels * = p < 0.05, ** = p < 0.01, *** = p < 0.001; y = years old; 6 Values in the same column with the same letter are not significantly different according to a One-Way analysis of variance where p = 0.05. Guatemalan rootstocks having fewer ripe rots than M x G or G x M rootstocks. The Guatemalan rootstock 'Reed' was both the best and worst rootstock with respect to fruit quality in each orchard depending on the year the fruit were sampled. There were similar general results on orchard E where the M x G rootstocks were better and worse than the G x M rootstock. Such results indicate that there are other influences on the levels of ripe rots 'Hass' fruit develop and that the rootstock of the tree is unlikely to have an overriding effect on fruit quality. 87
The results from this study do not rule out an important role that the rootstock may play on setting the incidence and severity of ripe rots on 'Hass' fruit after storage. As this study was preliminary, establishing the effect rootstocks have on fruit quality would require greater replication and more fruit samples to be evaluated from trees where the on orchard factors that influence rot levels in the ripe fruit are better controlled or known. However, based on the results reported here it is likely that the rootstock of the tree fruit are taken from will only partially explain differences in ripe rots of fruit from different trees in New Zealand avocado orchards. This is in contrast to the important role claimed for rootstocks in Australia where the race of the rootstock is considered to be closely related to the amount of postharvest rots 'Hass' avocado fruit develop (Whiley et al., 2007). In particular, the rootstock 'Zutano' has been claimed to result in 'Hass' fruit with poor postharvest performance on the basis of 'Zutano' having predominantly Mexican genes (Whiley et al., 2007). In this study 'Hass' fruit from trees with 'Zutano' seedling rootstock had the fewest ripe rots on three of the five orchards surveyed. The apparent variable influence of rootstock on fruit quality may be due to other as yet unknown orchard factors unrelated to the rootstock. CONCLUSIONS The rootstock of a 'Hass' avocado tree does not have an overriding influence on the amount of ripe rots 'Hass' avocado fruit develop after storage. There is other on orchard factors that are also likely to be important in determining levels of ripe rots. REFERENCES Dixon, J. (2003). New Zealand Avocado Fruit Assessment Manual. Avocado Industry Council Limited. Everett, K.R., Rees-Gearge, J., Parkes, S.L. and Johnston, P.R. (2003). Predicting avocado fruit rots by quantifying inoculum potential in the orchard before harvest. New Zealand Avocado Growers' Association Research Report 3: 93-98. Hofman, P.J., Vuthapanich, S., Whiley, A.W., Klieber, A. and Simons, D.H. (2002). Tree yield and fruit minerals concentrations influence 'Hass' avocado fruit quality. Scientia Horticulturae 92: 113-123. Lahav, E. and Whiley, A.W. (2001). Mineral nutrition and irrigation. In: The Avocado: Botany, Production and Uses (A.W. Whiley, B. Schaffer and B.N. Wolstenholme Eds.). CAB International, Wallingford, Oxon, United Kingdom pp. 15-37. Pak, H.A, Dixon, J. and Cutting, J.G.M. (2003). Impact of rainfall prior to harvest on ripe fruit quality of 'Hass' avocados in New Zealand. New Zealand Avocado Growers' Association Research Report 3: 22-31. Smith, D.G. (1993). Horticultural performance of imported cultivars and rootstocks in the northeastern Transvaal. South African Avocado Growers' Association Yearbook 16: 28-30. Thorp, T.G., Hutching, D., Lowe, T. and Marsh, K.B. (1997). Survey of fruit mineral concentrations and postharvest quality of New Zealand-grown 'Hass' avocado (Persea americana Mill.). New Zealand Journal of Crop and Horticultural Science 25: 251-260. Whiley A.W., Hofman P.J. and Coates L.M. (1997).Proceedings from Conference '97: Searching for quality (J.G.M. Cutting Ed.). Joint Meeting of the Australian Avocado Growers' Federation, Inc. and New Zealand Avocado Growers' Association, Inc., 23-26 September 1997 pp. 83-97. Everett, K.R. and Pak, H.A. (2001). Orchard survey: effect of pre-harvest factors on postharvest rots. New Zealand Avocado Growers' Association Research Report 1: 12-17. 88
Whiley, A., Giblin, F., Pegg, K. and Whiley, D. (2007). Preliminary results from avocado rootstock research in Australia. Proceedings of the VI World Avocado Congress. Paper 1a-7. Willingham, S.L., Pegg, K.G., Cooke, A.W., Coates, L.M., Langdon, P.W.B. and Dean, J.R. (2001). Rootstock influences postharvest anthracnose development in 'Hass' avocado. Australian Journal of Agricultural Research 52: 1017-1022. ACKNOWLEDGEMENTS The AIC wishes to thank Wally Colville, Sue and Rob Freeman, Hugh Moore, Stuart Pascoe, Les Blagrove and Ron Bailey for fruit. 89
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